Cytokinesis is the mechanical process by which a cell divides in two. The process is essential for propagation of unicellular parasites. Little is known about cytokinesis in African trypanosomes, but current information suggests that the process is different from that of the mammalian host. My laboratory has a strong interest in cytokinesis because it is clearly tied to signaling events. In particular, it utilizes temporal information to be triggered only at a specific time in the division cycle; and spatial information to form a cleavage furrow only in the appropriate site. One way to provide spatial and temporal information to the signal process is by means of signal anchor proteins. We have recently identified a putative signal anchor protein called TRACK with strong similarity to mammalian and yeast RACK1/Cpc2. We report that TRACK can correct morphological and growth related defects in cpc2 null mutants of S. pombe. In T. brucei, knock-down of TRACK with RNAi demonstrates that it is essential for progression through a mid-stage of cytokinesis. Cells that become "stuck" in cytokinesis continue through multiple rounds of partial division, and thus accumulate multiple nuclei, and multiple cytoplasmic extensions, each with attached flagellum. The observations are significant since they are the first to demonstrate that once cytokinesis is initiated, it is discontinuous and uses a different set of proteins at mid-stage than at the outset. Since no other RACK1/Cpc2 homologue has been implicated in cytokinesis, these data also underscore the unique aspects of the process in T. brucei. The disruption of TRACK blocks cell division, and indicates that novel components within the cytokinesis pathway may be useful as targets for therapy design. The current project utilizes TRACK as a tool to specifically probe molecular events at the mid-stage of cytokinesis. The project has three specific aims. The first aim examines the role of TRACK in cytokinesis through an examination of its binding partners. The second aim investigates proteins downstream of TRACK whose phosphorylation state changes in TRACKi knock-down cells. The third aim is to screen RNAi libraries for the unique phenotypic changes that accompany defective cytokinesis. In this way TRACK binding partners and other components of this essential process will be identified.